Errani, who boasts a top-speed serve of less than 90 mph couldn't hold once in six service games.

She was unable to capitalise on any of four break points in the opening game of the match and after fighting for that hold Wozniacki was rolling.

"It was really tricky conditions with the wind but I tried to stay aggressive," Wozniacki said, calling the opening game "very important".

"You always go out there and are a little bit nervous. I served really well after that."

Errani did break Wozniacki to open the second set, but with the Italian unable to protect her own serve it was immaterial, and the Dane wrapped things up in 65 minutes.

"It means so much to me," said Wozniacki, who has struggled to find consistency this season and coped with the abrupt and highly publicised end of her engagement to golfer Rory McIlroy.

"It's been a pretty up and down year for me, and to be here in the semifinals of the US Open again is just incredible."

Wozniacki toppled former champion Maria Sharapova in the fourth round, while Errani's run had included a third-round win over seven-time Grand Slam champion Venus Williams as upsets saw only two of the top 10 seeds reach the last eight.

She is the only woman besides two-time defending champion Serena Williams who has yet to drop a set in a tournament in which Williams and 10th-seeded Wozniacki are the only players remaining from the top 10 seeds.

Peng took control early against Bencic, the youngest US Open quarterfinalist since Martina Hingis won the title at 16 in 1997.

Peng fended off two break points in her opening service game and didn't face another.

She broke Bencic twice in the first set, and seized a 4-0 lead with two breaks of serve in the second.